CG-0MMJ8S90S1P2HW74: add versioned save/load infrastructure

[SorraTheOrc]

Summary

• add core engine save/load infrastructure (SaveLoadStore) with IndexedDB primary storage, localStorage fallback, domain separation (run-checkpoint vs campaign), and versioned serializer helpers
• export the save/load API from the core barrel so games can consume a stable engine surface for serialize/deserialize + backend operations
• add Main Street adapters for turn-start checkpoint and campaign progression persistence, including deterministic restore support (numericSeed + rngCalls) and integration tests for round-trip + incompatible-version rejection

Validation

• npm test
• npm run build